As currently employed in home and abroad, a oxygen saturation measuring apparatus (also known as oximeter) usually measures the oxygen saturation via spectrophotometry which comprises a transmission method and reflection method. Both methods are based on the Lambert-Beer law and light-scattering theory, and are carried out by making use of the difference of the photoabsorption coefficients between a deoxyhemoglobin and an oxyhemoglobin. The Lambert-Beer law is formulated as:I=I0e−εcd where:
I represents transmitted light intensity;
I0 represents incident light intensity;
C represents concentration of solution of the photic substance;
d represents photic path length of the solution;
ε represents photoabsorption coefficient of the substance; from the above equation, it is derived that:D=lnI0/I=εcd; where D is known as optical density. The formula reveals that there exists a correlation between the photoabsorption state of a substance and its concentration. In other words, the formula gives the very indication of the possibility of predicating internal tissue components from the photoabsorption state of the tissue.
Researchers further study the photoabsorption property of two main components, i.e., Hb (deoxyhemoglobin) and HbO2 (oxyhemoglobin), which are closely related to the oxygen saturation, and find remarkable difference therebetween. As shown in FIG. 2, the solid line represents the HbO2 photoabsorption coefficient curve, and the dotted line represents the Hb photoabsorption coefficient curve. It can be seen from FIG. 2 that at the visible red light range with wavelength of 660 nm, the photoabsorption coefficient of HbO2 is only 1/10 that of Hb; at the 805 nm infrared light range, Hb and HbO2 has a isosbestic point; and at the 940 nm infrared light range, the photoabsorption coefficient of HbO2 is greater than that of Hb.
                                          Sa            ⁢                                                  ⁢                          O              2                                =                                    Hb              ⁢                                                          ⁢                                                O                  2                                /                                  (                                      Hb                    +                                          Hb                      ⁢                                                                                          ⁢                                              O                        2                                                                              )                                                      =                                          c                1                            /                              (                                                      c                    1                                    +                                      c                    2                                                  )                                                    ,                            (        1        )                                                                                    D                ⁡                                  (                  660                  )                                            =                              ln                ⁢                                                                  ⁢                                                                            I                      0                                        ⁡                                          (                      660                      )                                                        /                                      I                    ⁡                                          (                      660                      )                                                                                                                                              =                              ln                ⁡                                  (                                                                                                              I                          0                                                ⁡                                                  (                          660                          )                                                                    /                                              I                        ⁡                                                  (                          660                          )                                                                                      ⁢                                          ⅇ                                                                        -                                                      ɛ                            1                                                                          ⁢                                                  c                          1                                                ⁢                        d                                                              ⁢                                          ⅇ                                                                        -                                                      ɛ                            2                                                                          ⁢                                                  c                          2                                                ⁢                        d                                                                              )                                                                                                                        =                                                                            ɛ                      1                                        ⁢                                          c                      1                                        ⁢                    d                                    +                                                            ɛ                      2                                        ⁢                                          c                      2                                        ⁢                    d                                                              ,                                                          (        2        )                                                                                    D                ⁡                                  (                  805                  )                                            =                              ln                ⁢                                                                  ⁢                                                                            I                      0                                        ⁡                                          (                      805                      )                                                        /                                      I                    ⁡                                          (                      805                      )                                                                                                                                              =                              ln                ⁡                                  (                                                                                                              I                          0                                                ⁡                                                  (                          805                          )                                                                    /                                              I                        ⁡                                                  (                          805                          )                                                                                      ⁢                                          ⅇ                                                                        -                                                      ɛ                            3                                                                          ⁢                                                  c                          1                                                ⁢                        d                                                              ⁢                                          ⅇ                                                                        -                                                      ɛ                            4                                                                          ⁢                                                  c                          2                                                ⁢                        d                                                                              )                                                                                                                        =                                                                            ɛ                      3                                        ⁢                                          c                      1                                        ⁢                    d                                    +                                                            ɛ                      4                                        ⁢                                          c                      2                                        ⁢                    d                                                              ,                                                          (        3        )            where SaO2 is the arterial oxygen saturation; c1 is HbO2 concentration, and c2 is Hb concentration; I0, I are incident light intensity and transmitted light intensity respectively; ε1, ε2 are the absorptivity of HbO2 and Hb to the red light of 660 nm wavelength respectively; ε3, ε4 are the absorptivity of HbO2 and Hb to the infrared light of 805 nm wavelength respectively, with ε3=ε4=ε; and d is the tissue transmission thickness. From (2) and (3) it is derived that:c1+c2=D(805)/εd, c1=(D(660)−ε2D(805)/ε)/(ε1−ε2)d; substituting them into (1):SaO2=A×D(660)/D(805)+B  (4)where, A=ε/(ε1−ε2), B=ε2/(ε1−ε2).
However, D(660) and D(805) are not correlated with Hb and HbO2 only, as expressed in the above equations (2) and (3). Rather, they are correlated with the absorption conditions of the muscle, skeleton, pigment, fats, venous blood and the like in the tissue. In other words, D(660), D(805) should contain a background absorption portion, as shown in FIG. 3. Thus (2) and (3) transform into:D(660)=lnI0(660)/I(660)=ln(I0(660)/IBe−ε1c1Δde−ε2c2Δd),  (5)D(805)=lnI0(805)/I(805)=ln(I0(805)/IBe−ε3c1Δde−ε4c2Δd),  (6)where I0 is the transmitted light intensity when only the tissue background absorption exits; Δd is the variation in the transmission range from absence of blood to abundance of blood. The background optic density DB may be simply defined as:DB=ln(I0/IB),thus:D(660)−DB(660)=ε1c1Δd+ε2c2Δd,  (7)D(805)−DB(805)=ε3c1Δd+ε4c2Δd,  (8)where ε3=ε4=ε, and (4) accordingly transforms into:SaO2=A×(D(660)−DB(660))/(D(805)−DB(805))+B;  (9)A and B are the same as above. Formula (9) is the basic formula for oxygen saturation measurement.
In a typical measurement, the infrared light wavelength usually adopted is about 940 nm, around which both the absorptivity of HbO2 and that of Hb vary flat, and the error is often relatively small. The basic calculation formula for oxygen saturation measurement is:Spo2=(A×R+B)/(C×R+D);  (10)where:
SpO2 is the oxygen saturation,
                              A          =                      ɛ            1                          ,                                  ⁢                  B          =                      -                          ɛ              2                                      ,                                  ⁢                  C          =                                    ɛ              4                        -                          ɛ              3                                      ,                                  ⁢                  D          =                                    ɛ              1                        -                          ɛ              2                                      ,                                  ⁢                              while            ⁢                                                  ⁢            R                    =                                                                      D                  ⁡                                      (                    660                    )                                                  -                                                      D                    B                                    ⁡                                      (                    660                    )                                                                                                D                  ⁡                                      (                    940                    )                                                  -                                                      D                    B                                    ⁡                                      (                    940                    )                                                                        .                                              (        11        )            
ε1, ε2 are the absorptivity of HbO2 and Hb to red light of 660 nm wavelength, respectively; ε3, ε4 are the absorptivity of HbO2 and Hb to infrared light of 940 nm wavelength, respectively.
From (11), it can be seen that R is in one-to-one correspondence with oxygen saturation, while D=LnI0/I=εcd,
                                          thus            ⁢                                                  ⁢            R                    =                                                                      ln                  ⁢                                                                          ⁢                                                            I                                              R                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      RM                                                                      -                                  ln                  ⁢                                                                          ⁢                                                            I                                              R                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      Rm                                                                                                                    ln                  ⁢                                                                          ⁢                                                            I                                              I                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      IM                                                                      -                                  ln                  ⁢                                                                          ⁢                                                            I                                              I                        ⁢                                                                                                  ⁢                        0                                                              /                                          I                      Im                                                                                            =                                          ln                ⁢                                                                  ⁢                                                      I                    Rm                                    /                                      I                    RM                                                                              ln                ⁢                                                                  ⁢                                                      I                    Im                                    /                                      I                    IM                                                                                      ;                            (        12        )            where, IRM is the maximum transmitted light intensity of the red light; IRm is the minimum transmitted light intensity of the red the optic signals with oxygen saturation information passing through the tissue terminals into an electric signals, and to digitalize the signals. With this numerical value, the oxygen saturation is then calculable by certain signal processing algorithm. FIG. 1 illustrates the block diagram of the structure of one particular embodiment in prior art, comprising: a power supply circuit, a photoelectric drive circuit, a signal amplification section, an A/D conversion circuit, a logic control section, a single chip processor, and a serial communication interface.
A pulse oximeter makes use of arterial blood pulsation in the human body tissue terminals, which is caused by the plethysmogram. The optical properties of the oxyhemoglobin and deoxyhemoglobin in the red light spectral region and the infrared region are different, which influences the red light and infrared light transmission of the finger. Hence, when the red light and the infrared light with a predetermined intensity are applied to the finger, by respectively detecting the transmitted light intensity of the two wavelengths, the oxyhemoglobin content can be calculated by the ratio between the optical density variations in the two light beams applied to the finger, so that the oxygen saturation may be calculated.
It can be derived from the above formula that when the luminotron is fixed, the formula (10) only has one variable R. The DC components of the red light and the infrared light (i.e., the DC components of the electric signals produced from the red light and light; IR0 is the incident light intensity of the red light; IlM is the maximum transmitted light intensity of the infrared light; Ilm is the minimum transmitted light intensity of the infrared light; Il0 is the incident light intensity of the infrared light.
As for the red light,
                                          ln            ⁢                                                  ⁢                                          I                Rm                            /                              I                RM                                              =                      ln            ⁡                          (                              1                -                                                                            I                      RM                                        -                                          I                      Rm                                                                            I                    RM                                                              )                                      ;                            (        13        )            when the AC quantity/DC quantity, i.e. (IRM−IRm)/IRM, is relatively small,
            ln      ⁡              (                  1          -                                                    I                RM                            -                              I                Rm                                                    I              RM                                      )              ≈                            I          RM                -                  I          Rm                            I        RM              ≈          AC      ⁢                          ⁢      quantity      ⁢              /            ⁢      DC      ⁢                          ⁢      quantity        ,thus R may be expressed in the following form:
  R  =                              Red          AC                /                  Red          DC                                      Ir          AC                /                  Ir          DC                      =                                        Red            AC                                Red            DC                          ·                              Ir            DC                                Ir            AC                              =                                    Ir            DC                                Red            DC                          ·                                            Red              AC                                      Ir              AC                                .                    
Therefore, the value of R can be calculated in case the waveform of one full plethysmogram of the two transmitted light beams is available.
The basic structure of the oxygen saturation measurement apparatus comprises a probe and a signal processing means, in which the probe is a sensor composed of a light-emitting diode and a light sensitive element. The light-emitting diode generates light with two or more wavelengths, usually the red light and the infrared light. The light sensitive element functions to convert the infrared light, similar hereinafter) remain relatively stable within a certain period, so the main factor influencing the value of R is the AC components of the red light and the infrared light (i.e., the AC components of the electric signals produced from the red light and the infrared light, similar hereinafter). Therefore the oxygen saturation can be calculated so long as the AC components of two light beams are available. The conventional method of calculating the ratio between the AC components is to determine the maximum value and the minimum value of the two light beams. However, such a method of calculating the AC components of the two light beams by determination of extreme values has the following drawbacks.
1) When the patient to be measured is in a state of anaesthesia, the plethysmogram of the patient may be very weak. In such a weak perfusion condition, even if there is no change in the luminescent intensity of the probe, irregular baseline drift often occurs to the received signal. Though such a drift is nonlinear, it can be approximated by the multi-order linear functions, and the approximation error is neglectable when the measurement precision is satisfied. Such a drift generates a very strong low frequency interference in the signal spectrum, while it generates a very strong interference with the pulse oxygen and pulse calculation when it is within the measuring frequency band so as to heavily influence the pulse oxygen calculation and lead to a great error.
2) When the patient is in a state of weak perfusion, the signal is very weak. Thus the signal-to-noise ratio is very low, the determination of the waveform at this point is very difficult. Since the determination of the crest and trough of the plethysmogram may result in error, the ratio of the AC component to DC component would be incorrect, and accordingly it is hard to ensure the accuracy of the pulse oxygen and pulse rate measurement.
3) While monitoring the pulse oxygen of a patient, the probe of the oximeter is usually bonded to the foot of the patient. However, the patient will move unconsciously, which is the case in particular with infants and the new-born babies. For instance, the fingers (or toes) of the patient often move, so that the distance between the fingers (or toes) and the sensor is changed, resulting in a very unstable plethysmogram waveform measured and a rather low signal-to-noise ratio. If the pulse oxygen is to be measured with a prior art method, it is necessary to measure the AC component of the plethysmogram. That is, the maximum value and the minimum value of the plethysmogram waveform should be located. However, in case of the presence of movement, the crest and trough of the plethysmogram waveform may be incorrectly determined and it is very difficult to obtain the accurate extreme values, so that the pulse oxygen measuring precision is very poor. As shown in FIG. 4, a moving condition will bring about, for example, baseline drift, impulsive noise, step noise and the like, to the plethysmogram, and at this moment it is very difficult to correctly determine the extreme values of the two light beams.